Every 5 years ABS demographers 'rebase' (or 'recalibrate') our estimated resident population (ERP) figures, to take advantage of information from the most recent Census. To rebase Australia's official population estimates, we use data from the Census and we make an adjustment for undercount (i.e. people missing from Census counts, as identified by the Census Post Enumeration Survey). We also make an adjustment for Australians who were temporarily overseas. From this the ABS produces the best estimate of the number of people who usually reside in Australia.

In 2011, the ABS improved our estimate of undercount, through a major change in our methodology (through the introduction of Automated Data Linking, which enabled us to better match people between the Census and the Census Post Enumeration Survey). As a result, we had a more accurate, but also lower estimate of how many people we needed to add to the 2011 Census counts when deriving our population estimates.

We also calculated the undercount using the previous method and estimated that if we had used that method we would have added in around 250,000 people who were not actually in the population.

From this analysis we have also determined that too many people were added to Census counts for earlier 'base' years (e.g. 2006), because the previous methodology overestimated the undercount. The ERP figures for these years are therefore higher than those calculated using the new methodology. Consequently, we need to revise historical population data to ensure population growth in recent years reflects what we know from births, deaths and migration information.

The ABS won't change any Census data through this process, just the undercount adjustments for the base years of 2006, 2001 and 1996. An indicative estimate of the size of this change 240,000 fewer people at 2006, 130,000 fewer at 2001 and 70,000 fewer at 1996.

Revised September 1991 to March 2011 population estimates for Australia and for all states and territories will be released in June 2013. Revised data for areas within each of the states and territories, such as local government areas, will be released in August 2013.

∙ Will data for my region change through this revision process?

Yes. All historical population estimates will be revised through this process. Information on the impact of these revisions will be included when data are released in June 2013 and August 2013.

∙ Will Census data be revised through this process?

No. An alternative undercount adjustment, from the Census Post Enumeration Survey, will be derived through this process and used to revise population estimates. Census data will not be revised.

∙ Is the ABS confident in the 2011 population estimates?

Yes. The ABS undertook extensive quality assurance processes before releasing the improved population estimates, to confirm the high quality of the 2011 population estimates. A similarly detailed quality assurance process will be used for the revised historical data.

∙ Will this affect other ABS data?

Yes. All data that references historical population estimates will be affected by this revision. An article in the June 2012 issue of Australian Demographic Statistics (cat. no. 3101.0), released on 18 December 2012, provides some examples. Further information will be included in the December 2012 issue, to be released on 20 June 2013.

∙ Where can I go to find out more about rebasing of population estimates?

The December 2011 issue of Australian Demographic Statistics (cat. no. 3101.0), released on 20 June 2012, includes a feature article which summarises how rebasing is undertaken. This publication can be found by clicking on the 'population clock' on the home page of the ABS website - www.abs.gov.au.

∙ What data should I use while waiting for the revisions to be published?

The ABS appreciates people will need to calculate growth measures before data are revised in 2013. The March 2012 issue of Australian Demographic Statistics (cat. no. 3101.0), released on 27 September 2012, included the feature article 'Advice on the use of 2011 preliminary rebased ERP'. This article provided advice on using preliminary data, until data could be revised in mid-2013.

Consider an example where a population figure for 30 June 2009 is required, consistent with the best estimate of growth for the period. Until revisions have been made, the ABS recommends using the components of growth (births, deaths and migration information) to work backwards from the 30 June 2011 estimate.

The estimate of the population for 30 June 2011 is 22,323,933.

Between 30 June 2009 and 30 June 2011 the ABS estimates that 587,933 births occurred, 287,710 deaths occurred and 366,335 more people migrated into Australia than migrated out of Australia.

The 'backcasted' estimate for 30 June 2009 is therefore derived by starting with the 22,323,933 at the 30 June 2011 and subtracting 666,558 people (based on births minus deaths plus migration) - resulting in an estimate of 21,657,375.

This estimate of 21,657,375 compares to the preliminary rebased figure for 30 June 2009 of 21,778,845 and the unrebased figure for 30 June 2009 of 21,951,736.

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